VLAN internal allocation policy

[Hasho]

Hi guys 

Whenever you create a L3 interface on the a switch with a "no switchport" command, the switch create an access port with an extended VLAN depending on the internal allocation policy. In the background, its just an access port with an SVI for that VLAN. I have two  directly connected switches with different internal allocation policy, one ascending and the other descending. with different VLAN allocated per L3 interface, i can somehow  ping across. Does this mean the frame tags are locally significant and the access port is removing the tag? What will happen if i had a trunk between them?

!
interface GigabitEthernet0/1
no switchport
ip address 10.10.10.1 255.255.255.0
negotiation auto
end

SWTC1#show vlan internal usage

VLAN Usage
---- --------------------
1006 GigabitEthernet0/1

!
interface GigabitEthernet0/1
no switchport
ip address 10.10.10.2 255.255.255.0
negotiation auto
end

SWTC2#show vlan internal usage

VLAN Usage
---- --------------------
4094 GigabitEthernet0/1

SWTC2#ping 10.10.10.1 source 10.10.10.2
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.10.10.1, timeout is 2 seconds:
Packet sent with a source address of 10.10.10.2
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 6/13/25 ms

[filopeter]

Hello,

when you create a routed interface, the native IOS handles creating the VLAN for you. Then it assigns the VLAN to that port and the MSFC or route processor. VLAN number is locally significant and the packet which leaves the "no switchport" interface is not 802.1Q tagged.

When you try to create a vlan after creating a routed interface with the same number as assigned to "no switchport" interface you end up with an error
!
switch(conf)#vlan 1006
switch(conf)#end
% failed to create VLANs 1006
!
the switch will not create this "no switchport" vlan in the vlan database and therefore it cannot be send over a trunk interface.

Best Regards,

Peter